This invention relates generally to the recovery of hydrogen fluoride (HF) from aqueous streams. More particularly it relates to a process for removing HF from dilute aqueous streams and recovering concentrated HF.
HF is widely used in industry. For example, owing to its very reactive nature it can be used to react with silica and therefore is employed in removal of sand from metal castings and in polishing or etching glass, it is also used to introduce fluorine atoms into hydrocarbons and polymers, for separating uranium isotopes, for flotation of ores, and in the petroleum and petrochemical industries as an alkylation catalyst. As might be expected, waste waters from such processes may contain small quantities of HF and they represent environmental cleanup problems. If HF could be removed from these streams, their disposal would be made easier and less expensive. However, if the HF could be recovered in a concentrated form, it could be reused.
The concentration of HF in water is made difficult by the fact that the two compounds are strongly bound together and an azeotrope is formed, having a concentration of about 38 wt. % HF and 62 wt. % water. Thus, it is not possible to simply distill a dilute solution of HF to concentrate it beyond that point. Methods to bypass the azeotropic concentration must be used, which involve the use of sulfuric acid to remove water or chemicals to complex HF. It would be particularly advantageous if HF could be concentrated without employing such methods.
The recovery of HF has been the subject of interest in many patents. For example, it has been proposed that HF be removed from aqueous solutions by an anion exchange resin and then displaced from the resin by another halogen hydride. Alternatively, it has been reported that HF may be extracted from solutions using a water-immiscible amine-containing extractant. Another method of recovering HF is to use electrodialysis.
In U.S. Pat. No. 4,902,312 recovery of small amounts of HF and HCl from chlorofluorocarbons by adsorption is disclosed to be feasible using carbon molecular sieves having a narrow pore size centered at 3.5 Angstroms. In examples of the process the HF is adsorbed from chlorodifluoromethane, which is a much larger size molecule. However, if such carbon molecular sieves are used to remove HF in the presence of water, the water molecules are not excluded by pore size and both molecules are adsorbed.
Thus, it is not immediately evident that a significant separation of HF and water would be possible. However, the inventors have found that it is possible to remove HF from aqueous solutions and also that surprisingly, it is possible to concentrate the HF when the carbon molecular sieves are regenerated.